The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories: spark ignition and compression ignition. Spark ignition engines, such as gasoline engines, introduce a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Compression ignition engines, such as diesel engines, introduce or inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient. In general, diesel engines are more efficient but produce more emissions.
Other types of combustion technologies have been introduced for internal combustion engines. One such technology is known in the art as the homogeneous charge compression ignition (HCCI). The HCCI combustion mode includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In a typical engine operating in the controlled auto-ignition combustion mode, the intake charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. Because controlled auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for controlled auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the controlled auto-ignition mode can operate unthrottled to achieve diesel-like fuel economy.
At medium engine speed and load operation, a combination of valve timing strategy and exhaust rebreathing (the use of exhaust gas to heat the cylinder charge entering a combustion space in order to encourage auto-ignition) during the intake stroke has been found to be very effective in providing adequate heating to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. This method, however, does not work satisfactorily at or near idle speed and load conditions. As the idle speed and load is approached from a medium speed and load condition, the exhaust temperature decreases. At near idle speed and load there is insufficient heat energy in the rebreathed exhaust to produce reliable auto-ignition. As a result, at the idle condition, the cycle-to-cycle variability of the combustion process is too high to allow stable combustion when operating in the HCCI mode. Consequently, one of the main issues in effectively operating an HCCI engine has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions. The benefits of HCCI combustion have been known for many years. The primary barrier to product implementation, however, has been the challenges of directly controlling the HCCI combustion process.
The HCCI engine is able to transition between operating in an auto-ignited combustion mode at part-load and lower engine speed conditions and in a conventional spark-ignited combustion mode at high load and high speed conditions. These two combustion modes require different engine operation to maintain robust combustion. For instance, in the auto-ignited combustion mode, the engine operates at lean air-fuel ratios with the throttle fully open to minimize engine pumping losses. In contrast, in the spark-ignition combustion mode, the throttle is controlled to restrict intake airflow and the engine is operated in at a stoichiometric air-fuel ratio.
In the typical HCCI engine, engine air flow is controlled by adjusting an intake throttle position, or adjusting opening and closing of intake valves and exhaust valves, using a variable valve actuation (VVA) system that includes a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift profiles. There is a need to have a smooth transition between these two combustion modes during ongoing engine operation, in order to prevent engine misfires or partial-burns during the transitions.
The combustion process in an HCCI engine depends strongly on factors such as cylinder charge composition, temperature, and pressure at the intake valve closing. Hence, the control inputs to the engine, for example, fuel mass and injection timing and intake/exhaust valve profile, must be carefully coordinated to ensure robust auto-ignition combustion. Generally speaking, for best fuel economy, an HCCI engine operates unthrottled and with a lean air-fuel mixture. Further, in an HCCI engine using exhaust recompression valve strategy, the cylinder charge temperature is controlled by trapping different amount of the hot residual gas from the previous cycle by varying the exhaust valve close timing. Typically, the HCCI engine is equipped with one or more cylinder pressure sensors and a cylinder pressure processing unit which samples cylinder pressure from the sensor and calculates the combustion parameters such as CA50 (location of 50% fuel mass burn), IMEP, and, NMEP, among other. The objective of HCCI combustion control is to maintain desired combustion phasing, indicated by CA50, by adjusting multiple inputs such as intake and exhaust valve timings, throttle position, EGR valve opening, injection timing, etc., in real-time. Thus, the cylinder pressure processing unit generally employs expensive, high-performance Digital Signal Processing (DSP) chips to process the vast amount of cylinder pressure samples to generate combustion parameters in real-time.
In an HCCI engine with multiple cylinders, combustion timing for each cylinder can vary significantly due to differences in the intake and thermal boundary conditions of each cylinder. It is known for a single cylinder engine, that adjusting both negative valve overlap (NVO) and combustion control parameters such as injection mass and timing, split fuel injection, spark timing, throttle and EGR valve positions combustion phasing control and robust HCCI combustion can be achieved using either a fully flexible valve actuation (FFVA) system or a simplified mechanical two-step with equal cam phasing. However, for multi-cylinder engines, throttle and EGR valve positions have global effects on combustion phasing for all cylinders, therefore, such combustion control parameters cannot be used for individual combustion phasing control. Likewise, a multi-cylinder engine equipped only with a conventional mechanical cam phasing system results in the same NVO applied to all cylinders and the capability of individual cylinder NVO control for combustion phasing is also not feasible.